For Karl Marx, ideological forms of consciousness are false, but how and in what respects? Ideologies must include some beliefs in order to be false, even if not all the beliefs that are inferentially related in the ideology are false, and even if there are (causally) related attitudes in the ideology that are neither true nor false. “Ideological” beliefs, however, are not simply false; their falsity has the specific property of not being in the interests of the agents who accept the ideology. One can make two kinds of mistakes about interests. One can mistake what is in one’s intrinsic interest or one can mistake what is in one’s extrinsic interest (that is, the means required to realize one’s intrinsic interests). Marx is mostly, but not exclusively, focused on mistakes about extrinsic interests; this is important in understanding how “morality” (which is not a matter of beliefs, but attitudes) can be ideological for Marx. I illustrate this analysis with some of Marx’s paradigmatic examples of ideological mistakes and offer an account of Marx’s conception of “interests.”